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Today's Topics:
1. Re: monad nomad gonad gomad (prad)
2. Re: global variables (prad)
3. Re: monad nomad gonad gomad (Ertugrul Soeylemez)
4. Re: Re: global variables (Brandon S Allbery KF8NH)
5. Re: Re: global variables (Ozgur Akgun)
6. Re: global variables (Ertugrul Soeylemez)
7. Re: Re: monad nomad gonad gomad (Thomas)
----------------------------------------------------------------------
Message: 1
Date: Sun, 15 Aug 2010 10:52:26 -0700
From: prad <p...@towardsfreedom.com>
Subject: [Haskell-beginners] Re: monad nomad gonad gomad
To: beginners@haskell.org
Message-ID: <20100815105226.33c50...@gom>
Content-Type: text/plain; charset=US-ASCII
On Sun, 15 Aug 2010 16:43:09 +0200
Ertugrul Soeylemez <e...@ertes.de> wrote:
> I'm not sure whether you are reading our posts, but we have
> explained in detail the difference. Once understood, you won't get
> errors like that anymore.
>
> As a well meant suggestion, you have to actually read the answers to
> your posts, even if they are long.
>
ertugrul,
believe me i read the posts (more than a few times) - especially yours
in fact and am very appreciative. however, it takes me a while to
understand what's going on - even to connect similar ideas together. i
certainly don't mind long posts either, but i am still having
difficulties putting the ideas together. i tend to be a bit slow anyway
and keep thinking in python terms.
i intend to respond once i understand the ideas better. for instance,
what you wrote here made sense to me:
========
let readBlahTxt = readFile "blah.txt"
content <- readBlahTxt
This should show the difference more clearly.
========
however, i still can't quite connect it to the idea of what it has to
do with getting monad data out of a monad and into the pure realm.
late last night, though, i was able to do so, but it took while.
> Posting essentially the same questions over and over gets you nowhere.
>
agreed and i apologize. my problem is that i don't even see that it is
the same question. as you have pointed out once before, "You're still
thinking way too complicated."
i think what happens is i keep trying different things over and over
again. that affects my thinking no doubt. i should likely think more
and try less.
> A monad is just a way to combine computations.
>
this wasn't clear to me till now even though i had read your continuing
exchange with kyle and the example both of you were discussing
regarding >>= (and i had read up on the operator and even tried it).
> I don't like to mention my own one
>
i would very much like to see yours.
> I agree with Brent
>
i think brent's delightful blog post describes my present dilemmas ...
except i haven't quite gotten to the point of being able to say "monads
are burritos" ... so i think there is likely still hope for me. ;)
--
In friendship,
prad
... with you on your journey
Towards Freedom
http://www.towardsfreedom.com (website)
Information, Inspiration, Imagination - truly a site for soaring I's
------------------------------
Message: 2
Date: Sun, 15 Aug 2010 11:04:11 -0700
From: prad <p...@towardsfreedom.com>
Subject: [Haskell-beginners] Re: global variables
To: beginners@haskell.org
Message-ID: <20100815110411.1daa5...@gom>
Content-Type: text/plain; charset=US-ASCII
On Wed, 11 Aug 2010 22:25:45 +0200
Ertugrul Soeylemez <e...@ertes.de> wrote:
> My way of doing this is the following (snippet):
>
> import Control.Applicative
> import MonadLib
>
> type DatabaseT = ReaderT Connection
>
> myApplication :: DatabaseT IO ()
> myApplication = do
> dbConn <- ask
> inBase $ runRaw dbConn "SELECT now();"
>
> runDatabaseT :: BaseM m IO => DatabaseT m a -> m a
> runDatabaseT c = do
> conn <- inBase $ connectPostgreSQL ""
> runReaderT (Config conn) c
>
> main :: IO ()
> main = runDatabaseT myApplication
>
thank you ertugrul. i understand the explanations you and MAN have
provided regarding global variables.
however, i don't understand what you have done above despite the
illustrative game example. i've been in the middle of trying to get
info out of a monad (as indicated in other posts) and have finally
gotten by the errors (although the understanding hasn't quite congealed
yet). as a result i haven't been able to try to incorporate the above
yet (which is why i haven't responded till now).
i did note that MonadLib doesn't seem to exist, so is that something i
get from cabal? is ask part of it too?
> What you want is not a global variable, but a convenient way to pass
> environment values to functions and computations.
>
yes that is what i want, but i haven't gotten the gist of the code you
have provided me with yet. i need to understand types better for one
thing.
> I hope this helps.
>
it will. i've flagged your post and will come back to it after
acquiring a bit more competence - as well as repetitive readings.
--
In friendship,
prad
... with you on your journey
Towards Freedom
http://www.towardsfreedom.com (website)
Information, Inspiration, Imagination - truly a site for soaring I's
------------------------------
Message: 3
Date: Sun, 15 Aug 2010 21:11:52 +0200
From: Ertugrul Soeylemez <e...@ertes.de>
Subject: [Haskell-beginners] Re: monad nomad gonad gomad
To: beginners@haskell.org
Message-ID: <20100815211152.4ad15...@tritium.streitmacht.eu>
Content-Type: text/plain; charset=US-ASCII
prad <p...@towardsfreedom.com> wrote:
> On Sun, 15 Aug 2010 16:43:09 +0200
> Ertugrul Soeylemez <e...@ertes.de> wrote:
>
> > I'm not sure whether you are reading our posts, but we have
> > explained in detail the difference. Once understood, you won't get
> > errors like that anymore.
> >
> > As a well meant suggestion, you have to actually read the answers to
> > your posts, even if they are long.
>
> believe me i read the posts (more than a few times) - especially yours
> in fact and am very appreciative. however, it takes me a while to
> understand what's going on - even to connect similar ideas together. i
> certainly don't mind long posts either, but i am still having
> difficulties putting the ideas together. i tend to be a bit slow
> anyway and keep thinking in python terms.
>
> i intend to respond once i understand the ideas better. for instance,
> what you wrote here made sense to me:
>
> ========
> let readBlahTxt = readFile "blah.txt"
> content <- readBlahTxt
>
> This should show the difference more clearly.
> ========
>
> however, i still can't quite connect it to the idea of what it has to
> do with getting monad data out of a monad and into the pure realm.
> late last night, though, i was able to do so, but it took while.
I'd say don't bother. "Getting out of" is just a metaphor. Monads are
no boxes. You have a monadic computation, which has a result, and you
want to refer to this result, and that's all.
let a = getLine
b <- getLine
In this example, 'a' is the same as 'getLine'. It's a computation,
which reads a line from stdin. It's not the result of the computation
(String), but the computation itself (IO String), because you defined
'a' and 'getLine' to be the same thing. On the other hand, 'b' is the
result. You could just as well write:
b <- a
Just remember this: You have an IO computation? You want to refer to
its result? Fine, just use '<-'. That's it.
Now what is meant by the "getting out of" metaphor? You can use the
'<-' notation only inside of a monadic computation. In other words,
even though you can refer to the result of getLine, ultimately you are
making a new IO computation, which uses the result:
readAndSquare :: IO Integer
readAndSquare = do
line <- getLine
let n = read line
return (n^2)
readAndSquare is an IO computation itself and there is nothing you could
do about it. As long as you refer to results of IO computations, if you
go down the call tree, you will always end up in an IO computation.
This is what people refer to as not being able to "get out of" IO. This
is not true for the list monad:
getFirstWord :: String -> String
getFirstWord = head . words
The list concept is also a monad, or in Haskell terms: [] is a monad.
You are using the result of a monadic computation, namely 'words':
words :: String -> [String] -- brackets style
words :: String -> [] String -- prefix style
But the getFirstWord function is itself not monadic (let's disregard the
fact that strings are lists). Using the 'head' function
head :: [a] -> a -- brackets style
head :: [] a -> a -- prefix style
you have broken out of the [] monad. There is no such function for the
IO monad:
breakOut :: IO a -> a
Well, that's not entirely true. There is unsafePerformIO, which has
exactly that type signature, but seriously, never use it. Especially as
a beginner coming from the imperative world one is often tempted to use
it, but don't. Using it is almost always the wrong approach, and as its
name suggests a very unsafe one, too.
> > Posting essentially the same questions over and over gets you
> > nowhere.
>
> agreed and i apologize. my problem is that i don't even see that it is
> the same question. as you have pointed out once before, "You're still
> thinking way too complicated."
>
> i think what happens is i keep trying different things over and over
> again. that affects my thinking no doubt. i should likely think more
> and try less.
In fact, I recommend not to think more, but to think less. Forget the
word 'monad' and just concentrate on specific monads like IO or Maybe.
At some point it will make click and suddenly everything gets clear.
Everybody has this experience with monads (unless they give up).
> > I don't like to mention my own one
>
> i would very much like to see yours.
It's linked on the tutorials wiki page as "Understanding Haskell
Monads".
> > I agree with Brent
>
> i think brent's delightful blog post describes my present dilemmas ...
> except i haven't quite gotten to the point of being able to say
> "monads are burritos" ... so i think there is likely still hope for
> me. ;)
Try this [1]. ;)
[1] http://blog.plover.com/prog/burritos.html
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
------------------------------
Message: 4
Date: Sun, 15 Aug 2010 15:42:25 -0400
From: Brandon S Allbery KF8NH <allb...@ece.cmu.edu>
Subject: Re: [Haskell-beginners] Re: global variables
To: beginners@haskell.org
Message-ID: <4c684321.5080...@ece.cmu.edu>
Content-Type: text/plain; charset=UTF-8
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
On 8/15/10 14:04 , prad wrote:
> i did note that MonadLib doesn't seem to exist, so is that something i
> get from cabal? is ask part of it too?
MonadLib is on Hackage, yes. "ask" is part of the MonadReader typeclass.
- --
brandon s. allbery [linux,solaris,freebsd,perl] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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------------------------------
Message: 5
Date: Sun, 15 Aug 2010 20:50:03 +0100
From: Ozgur Akgun <ozgurak...@gmail.com>
Subject: Re: [Haskell-beginners] Re: global variables
To: Brandon S Allbery KF8NH <allb...@ece.cmu.edu>
Cc: beginners@haskell.org
Message-ID:
<aanlktin4s3lqpx=pbzvwv+1acjjx4lk6hwvos2nmj...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On 15 August 2010 20:42, Brandon S Allbery KF8NH <allb...@ece.cmu.edu>wrote:
> On 8/15/10 14:04 , prad wrote:
> > is ask part of it too?
>
> "ask" is part of the MonadReader typeclass.
>
>
as a side note, asking hoogle might help in such questions:
http://www.haskell.org/hoogle/?hoogle=ask
hth,
Ozgur
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------------------------------
Message: 6
Date: Sun, 15 Aug 2010 22:34:19 +0200
From: Ertugrul Soeylemez <e...@ertes.de>
Subject: [Haskell-beginners] Re: global variables
To: beginners@haskell.org
Message-ID: <20100815223419.544ad...@tritium.streitmacht.eu>
Content-Type: text/plain; charset=US-ASCII
Brandon S Allbery KF8NH <allb...@ece.cmu.edu> wrote:
> On 8/15/10 14:04 , prad wrote:
> > i did note that MonadLib doesn't seem to exist, so is that something
> > i get from cabal? is ask part of it too?
>
> MonadLib is on Hackage, yes. "ask" is part of the MonadReader
> typeclass.
In MonadLib the class is named ReaderM, not MonadReader.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
------------------------------
Message: 7
Date: Mon, 16 Aug 2010 18:35:22 +0200
From: Thomas <hask...@phirho.com>
Subject: Re: [Haskell-beginners] Re: monad nomad gonad gomad
To: beginners@haskell.org
Message-ID: <4c6968ca.3000...@phirho.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi prad!
Reading the recent threads about Monads I decided to put my learning
experience online. It might be of use to someone (maybe you?).
http://codingcactus.wordpress.com/2010/08/16/writemonad1/
Regards,
Thomas
On 15.08.2010 19:52, prad wrote:
> On Sun, 15 Aug 2010 16:43:09 +0200
> Ertugrul Soeylemez<e...@ertes.de> wrote:
>
>> I'm not sure whether you are reading our posts, but we have
>> explained in detail the difference. Once understood, you won't get
>> errors like that anymore.
>>
>> As a well meant suggestion, you have to actually read the answers to
>> your posts, even if they are long.
>>
> ertugrul,
>
> believe me i read the posts (more than a few times) - especially yours
> in fact and am very appreciative. however, it takes me a while to
> understand what's going on - even to connect similar ideas together. i
> certainly don't mind long posts either, but i am still having
> difficulties putting the ideas together. i tend to be a bit slow anyway
> and keep thinking in python terms.
>
> i intend to respond once i understand the ideas better. for instance,
> what you wrote here made sense to me:
>
>
> ========
> let readBlahTxt = readFile "blah.txt"
> content<- readBlahTxt
>
> This should show the difference more clearly.
> ========
>
> however, i still can't quite connect it to the idea of what it has to
> do with getting monad data out of a monad and into the pure realm.
> late last night, though, i was able to do so, but it took while.
>
>
>> Posting essentially the same questions over and over gets you nowhere.
>>
> agreed and i apologize. my problem is that i don't even see that it is
> the same question. as you have pointed out once before, "You're still
> thinking way too complicated."
>
> i think what happens is i keep trying different things over and over
> again. that affects my thinking no doubt. i should likely think more
> and try less.
>
>
>> A monad is just a way to combine computations.
>>
> this wasn't clear to me till now even though i had read your continuing
> exchange with kyle and the example both of you were discussing
> regarding>>= (and i had read up on the operator and even tried it).
>
>
>> I don't like to mention my own one
>>
> i would very much like to see yours.
>
>
>> I agree with Brent
>>
> i think brent's delightful blog post describes my present dilemmas ...
> except i haven't quite gotten to the point of being able to say "monads
> are burritos" ... so i think there is likely still hope for me. ;)
>
------------------------------
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